Nobel Physics Prize goes to 2 Japanese, Canadian scientists

Takaaki Kajita of Japan and Arthur McDonald of Canada were awarded the Nobel Physics Prize yesterday for determining that neutrinos have mass, a key piece of the puzzle in understanding the cosmos.

"The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the Universe," the Royal Swedish Academy of Sciences said.

The findings are so far-reaching that they challenge the so-called Standard Model, the conceptual model of fundamental particles and forces, it said.

Neutrinos are lightweight neutral particles that are created as the result of nuclear reactions, such as the process that makes the Sun shine.

Next to particles of light called photons, they are the most abundant particles in the Universe.

Their existence was tentatively proposed in 1930, but was only proved in the 1950s, when nuclear reactors began to produce streams of the particles.

The prevailing theory was that neutrinos were massless, but experiments carried out separately in underground labs by teams led by Kajita in Japan and McDonald in Canada showed that this was not the case.

Many neutrinos blasted out from the Sun -- a type called electron neutrinos -- "oscillated" en route to become cousin particles called muon-neutrinos and tau-neutrinos, they found.

Since the 1960s, scientists had estimated the number of neutrinos created in the nuclear reactions that make the Sun shine.

But when this figure was compared against actual measurements on Earth, an anomaly emerged.

Up to two-thirds of the calculated tally of neutrinos coming from the Sun was missing, and no one knew where they were going.

In 1998, working at the Super-Kamiokande detector -- a 50,000-tonne tank of highly purified water built at the bottom of an old zinc mine in central Japan -- Kajita discovered that neutrinos seemed to change identities on their way from the Sun to Earth.

Meanwhile, in 1999, scientists led by McDonald at the Sudbury Neutrino Observatory, built deep under the ground in an old nickel mine in Ontario, Canada, were also studying neutrinos coming from the Sun. In 2001, his group also proved that neutrinos had a chameleon-like nature.

Under the quirky rules of quantum physics, the identity shift the scientists observed can only happen if neutrinos have mass.